In recent years, a liquid crystal display that has a light weight and consumes a small amount of electric power has been used as a most competitive display that can be used instead of a cathode ray tube. In particular, since a thin film transistor liquid crystal display (TFT-LCD) that is driven by using a thin film transistor independently drives each of pixels, a response speed of the liquid crystal is very high, thus, a high-quality dynamic image can be realized. Accordingly, currently, the thin film transistor liquid crystal display is applied to a notebook computer, a wall-mounted television and the like, and the application range thereof is expanded.
During the production of a typical color thin film transistor-liquid crystal display, a thin film transistor driving device and an ITO transparent electrode are layered on a glass substrate, and an alignment film is then layered thereon to form a lower substrate of a cell. Spacers are formed by using a silant in order to inject a liquid crystal material between inner surfaces of a pair of upper and lower substrates, polarized films are provided on outer surfaces of the glass substrates, and the liquid crystal material is injected between a pair of substrates and cured to produce a liquid crystal display cell.
In the TFT-LCD, in order to use the liquid crystal as an optical switch, it is required that the liquid crystal is initially aligned on the layer on which the thin film transistor is formed at the innermost part of the display cell in a predetermined direction. In order to achieve this, a liquid crystal alignment film is used.
As a method of preparing the alignment film, a rubbing treatment method of unidirectionally rubbing a polymer resin film made of a polyimide or the like formed on a substrate by using clothes or a method of inclinedly depositing silicon dioxide (SiO2) is known. In the case of the alignment film that is prepared by using the rubbing treatment method, there are problems in that the contamination is caused by the impurity that may be generated due to contact during the rubbing, the yield of the products is reduced due to the occurrence of static electricity, and contrast is reduced. In the case of the method of inclinedly depositing silicon dioxide, there are problems in that the preparing cost is increased and it is difficult to form the film having a large area, thus, the film is not suitable to be applied to a large liquid crystal display.
In order to solve this, an alignment method by a non-rubbing process using a photopolymerizable alignment material is developed to perform a photopolymerization by using the radiation of light so that the alignment of polymer is induced to align liquid crystals. A representative example of the non-rubbing process is an optical alignment using photopolymerization that is announced by M. Schadt, et al. (Jpn. J. Appl. Phys., Vol 31, 1992, 2155), Dae S. Kang, et al. (U.S. Pat. No. 5,464,669), and Yuriy Reznikov (Jpn. J. Appl. Phys. Vol. 34, 1995, L1000). The optical alignment is a mechanism in which a photoreaction of a photosensitive group that is connected to the polymer occurs due to linearly polarized ultraviolet rays, and in this procedure, a main chain of the polymer is unidirectionally aligned, thereby aligning the liquid crystals.
The polycinnamate-based polymer such as PVCN (poly(vinyl cinnamate)) and PVMC (poly(vinyl methoxycinnamate)) has been mainly used as a representative material of the photopolymerizable alignment film. However, the polycinnamate-based polymer has a problem in that the optical alignment property of the polymer is excellent but the thermal stability is poor. That is, the thermal stability of the alignment film depends on the thermal stability of the polymer, and since the main chain of the polymer of polyvinyl cinnamate has a glass transition temperature of 100° C. or less, there is a problem in that the thermal stability of the alignment film is reduced.
Meanwhile, Japanese Unexamined Patent Application Publication No. 11-181127 discloses a method of producing a polymer type of alignment film that has a main chain such as acrylate and methacrylate and a side chain having a photosensitive group such as a cinnamate group, and an alignment film that is produced by using the method. However, the patent is disadvantageous in that since the mobility of the polymer is poor, even though the polymer is exposed to light for a long time, it is difficult to obtain the desired alignment property. The reason for this is that since the photosensitive group which is present in the polymer is restricted by the main chain of the polymer, the group is difficult to rapidly react with the radiated polarized light. Accordingly, since a long time is required to obtain a network polymer, a process efficiency is reduced, and if an alignment treatment process is finished after an insufficient time, since the alignment of the liquid crystals is insufficient in the prepared liquid crystal display, there are problems in that a dichroic ratio is low and contrast is reduced.
Korean Unexamined Patent Application Publication Nos. 2006-0029068 and 2004-0102862 disclose that polarized UV is radiated on a coated liquid crystal material without using a rubbing process to determine an alignment direction of liquid crystal. However, as described in the above patent, in the case of when the polarized UV is radiated on a curable liquid crystal material to align the liquid crystal, since the curing of the liquid crystal occurs in an alignment direction, the insufficient curing occurs, thus reducing the surface strength and easily causing the deformation due to external impact or heat.
Accordingly, a demand for a novel photoreactive polymer that has the excellent thermal stability and the improved surface strength and photoreaction rate is growing.